1. Field of the Invention
The present invention relates to a spectrometric method capable of removing error factors.
2. Description of Related Art
Generally speaking, in the spectrometric method for determining a physical or chemical quantity photometric measurement of a target is made at plural wavelengths different from each other and the quantity is determined by a predetermined calibration curve using those data obtained by the photometric measurement.
The calibration curve is usually determined in the following manner.
Assuming the number of wavelengths used for the photometric measurement is n, n or more samples having known with respect to the quantity are prepared and the measurement is performed for all samples prepared. Further, assuming the calibration curve is represented by a linear combination of n photometric data obtained at n wavelengths in the following form; EQU b=Q.sub.1 B.sub.1 +Q.sub.2 B.sub.2 + . . . +QnBn
wherein b indicates the quantity to be determined, B.sub.1 to Bn are photometric data obtained at n wavelengths and Q.sub.1 to Qn are coefficients, these coefficients are determined according to a known approximation method such as least square method so that the sum of differences .DELTA.b.sub.i for i=1 to n each of which is defined as a difference between each of known values of the quantity and each of values thereof calculated according to the above equation can be minimized.
Once the calibration curve has been determined, the quantity of an unknown sample is calculated from the calibration curve using n photometric data at n wavelengths obtained with respect to the unknown sample.
Photometric data obtained include various errors. According it is necessary to remove those errors in order to obtain a correct value. As factors which cause those errors, temperature variation of the sample (hereinafter referred to as sample temperature variation), variation of the scattering factor of the sample (hereinafter referred to as sample scattering variation) and temperature variation of the measuring apparatus (hereinafter referred to as machine temperature variation) can be recited.
In order to remove errors due to the sample, measured data are corrected according to the temperature of sample measured at that time. Also, it is necessary to maintain the temperature of sample constant in order to avoid a rise of the temperature of sample due to the incident light during the measurement.
However, there may be a shift between the measured temperature and an actual temperature of sample and, also, there may be a difference between the temperature of a portion of the sample at which it is measured and the temperature of another portion thereof at which the photometric measurement is performed actually. Further, the spectrometer becomes more complicated and higher at the cost thereof in order to control the temperature of sample constant and, generally speaking, the temperature control itself is quite difficult.
In order to eliminate errors due to temperature drifts of the light source and sensors used in the spectrometer, data can be corrected according to changes in temperatures of them in a manner similar to the correction of errors due to the sample temperature variation and it is desirable to control the machine temperature in the spectrometer.
However, this is also quite difficult as in the case of the control of the sample temperature.
Further, for instance, the two wavelength spectrometry is used in order to eliminate errors caused by the sample scattering variation due to inhomogeneousness of the sample such as muddiness.
In this method, the measurement is performed using two lights of different wavelengths .lambda..sub.1 and .lambda..sub.2 to obtain a difference .DELTA.A between two absorbances A(.lambda..sub.1) and A(.lambda..sub.2) measured. If two wavelengths .lambda..sub.1 and .lambda..sub.2 are near with each other in the case of the sample having muddiness therein, errors due to the sample scattering variation are considered to be substantially equal between two wavelengths .lambda..sub.1 and .lambda..sub.2. Accordingly, the difference of absorbance .DELTA.A can be considered free from errors due thereto. Therefore, the determination of the quantity free from those errors can be made by using a calibration curve obtained from data regarding the difference of absorbances .DELTA.A.
Further, in the differential spectrometry, a calibration curve is determined using differential absorbance .DELTA.A obtained when the difference between two wavelength .lambda..sub.1 and .lambda..sub.2 is made close to zero. In this method, it is possible to remove errors due to the sample scattering variation which has no wavelength dependency.
However, in the known method such as the two wavelength spectrometry or the differential spectrometry, only the sample scattering variation with no wavelength dependency can be removed. In fact, the sample scattering varies with the wavelength as in Rayleigh scattering. Accordingly, it is impossible to remove errors due to the sample scattering variation completely.